Optical fiber connectors and splices are an essential part of optical fiber communications systems. Connectors may be used to join lengths of optical fiber into longer lengths, or to connect optical fiber to active devices such as radiation sources, detectors, or repeaters, or to passive devices such as switches or attenuators.
An optical fiber connector must meet at least two requirements. It must couple or join two optical fibers with minimum insertion loss. Secondly, it must provide mechanical stability and protection to the junction between the optical fibers in the working environment. Achieving low insertion loss in coupling two optical fibers in generally a function of the alignment of the optical fiber ends, the width of the gap between the ends, and the optical surface condition of the ends. Stability and junction protection is generally a function of connector design, such as, for example, the minimization of differential thermal expansion effects.
Many approaches to achieving fiber alignment can be found in the prior art. Among them are V-grooves, resilient ferrules, and conical bushings. A discussion of prior art connectors is provided in R. Schultz, Proceedings of the Optical Fiber Conference, Los Angeles (September 1982), pp. 165-170.
Some prior art optical fiber connectors contain one or more precision-machined parts and therefore are relatively costly items. Whereas this may be acceptable for some applications, in other cases the cost of such prior art connectors might constitute a significant fraction of the total installation cost. Thus, strong incentives exist for providing optical fiber connectors that do not require expensive precision-machined parts.
A further consideration in connector design is the relative ease of field installation of the connector. It is desirable that a sought-after connector be capable of being installed within a relatively short period of time without requiring special skills or manipulations not easily carried out in the field.
A prior art connector which has many of the above-listed desirable features includes two drawn glass cylindrical plugs, with a fiber end portion inserted into a close-fitting passageway of each plug, and the connection between the two fiber ends made by inserting the plugs in end-to-end fashion into an alignment sleeve that maintains the outer surfaces of the two plugs in registry. This connector design relies on the capability of producing plugs to very close tolerances by drawing them from a glass preform. Relative rotation of the two plugs typically changes the relative position of the fibers held within the passage way because of the eccentricity of the optical fiber core with respect to the plug. Eccentricity is defined as the distance between the longitudinal centroidal axis of the plug at an end face of the plug and the centroidal axis of the optical fiber core held within the passageway of the plug. Generally, the passageway is not concentric with the outer cylindrical surface which is the reference surface. Also, the optical fiber may not be centered within the plug passageway and the fiber core may not be concentric with the outer surface of the fiber. Hence, the eccentricity is comprised of the eccentricity of the optical fiber core within the optical fiber, the eccentricity of the optical fiber within the plug passageway and the eccentricity of the passageway within the plug;
Because it is very difficult to control the eccentricity of the optical fiber core in the plug in which it is terminated, it is difficult to achieve desired losses of 0.1 dB or less in single mode fibers without maintaining close tolerances so that the opposed cores are aligned to within about 0.7 .mu.m. This, of course, increases manufacturing costs.
Another prior art connector that has many of the above-listed desirable characteristics is disclosed in U.S. Pat. No. 4,545,644 which issued on Oct. 8, 1985 in the names of G. F. DeVeau, Jr. and C. M. Miller. That patent discloses an optical fiber connector comprising two cylindrical plugs with axial passageways into which the optical fiber end portions are inserted, with the plugs then inserted into an alignment device. The alignment device comprises a plurality, typically three, of cylindrical alignment rods, and facilities such as a spring clip for maintaining the alignment rods in contacting relationship with both plugs. At least one of the alignment rods is provided with a "flat" region of different curvature extending from one of the ends of the rod towards the middle, where a small amount of rod material has been removed to create a small offset. One or more flat-caring alignment rods can be used to introduce deliberately an eccentricity into the plug alignment. The rotation of one plug with respect to the other eliminates substantially misalignment between the fiber cores. Relative rotation between the two cylindrical plugs changes the relative position of the optical fiber end portions held within them.
If the total eccentricities of the two optical fiber ends to be joined are identical or at least very nearly so, then a low-loss connection can be achieved by merely rotating, within the alignment sleeve, one plug with respect to the other, until maximum coupling is observed. This is very often possible with mated cylindrical plugs originating from adjacent segments of the same drawn glass tubular stock. In U.S. Pat. No. 4,691,986 which is issued on Sep. 9, 1987 in the names of Aberson, et al., two plugs are made by a process that includes dividing a length of tubular stock into a plurality of segments, each segment corresponding to a plug. Contiguous end faces of two segments along the stock become opposed end faces for two plugs. Facilities are provided so that after optical fibers have been terminated by two contiguous plugs, the plugs are capable of being assembled in their preseparated relative positions. This arrangement has been referred to as a prealigned rotary splice.
Central to the so-called prealigned rotary splice is the recognition that eccentricity between plug passageway and plug cylindrical surfaces essentially will have no effect on alignment of fibers terminated by two plugs if the two plugs have essentially the same amount of passageway eccentricity relative to the cylindrical surfaces and if the plugs are aligned such that the eccentricities are in the same radial direction from a centroidal axis of the plugs. This is achieved if contiguous plugs are arranged so that the contiguous faces prior to separation from the tubular stock become the end faces of the plugs, and if the plugs are aligned rotationally to have substantially the same angular relationship that existed between the two contiguous segments prior to their separation. However, even with mated pairs it has not always been possible to achieve connections having losses less than 0.1 dB, because achievement of such a low-loss level typically requires alignment of the fiber ends to within less than about 1 .mu.m.
With the use of the prealigned rotary splice, loss due to eccentricity of the bore with respect to the plug has been overcome substantially. Also, by means of the rotary splice, the problem of loss due to offset of optical fiber cores has been overcome. Remaining is the loss due to random disposition of the optical fiber end portion in the passageway of the plug. This is overcome to a large extent by making the plug so that the transverse cross-section of the passageway therein is equal substantially to the transverse cross section of the optical fiber to be received therein. Of course, this requires precision plug manufacture and higher costs than if the cross section of the plug passageway was not so critical.
What is desired and seemingly what is not available in the prior art is an optical fiber connector in which the loss due to random positioning of the optical fiber in the plug passageway is overcome. If this problem were to be overcome, it is believed that any loss of optical fiber connections through the use of the prealigned rotary splice would be reduced substantially, possibly to a level as low as 0.05 dB.